In the past few years it has become more and more clear in the field of centrifugal pumps, which covers a relatively wide range of application in industry, that for the achievement of application-oriented maximum service lives the essential wearing parts of the pump must be made from materials which guarantee the correspondingly optimimum material properties with regard to the delivery medium concerned. Apart from the consideration of the flow rates of the delivery media through the pump, which are usually very high, other criteria mainly regarding the causes of damage leading to the destruction of the materials are of importance to this correspondingly individual selection of materials which must be adapted to the most different delivery media. These criteria can be substantially subsumed under the influences to be taken into account here, namely corrosion, erosion, cavitation and/or a combination thereof, the classification depending on the predetermination of specific parameters of the delivery media, especially the pH-value, the solids content, the temperature and the flow rate of the media.
When being employed, for instance in connection with highly aggressive delivery media, the materials which have mostly been used for heavily loaded centrifugal pumps when the aforementioned criteria are to be complied with are a martensitic chromium-molybdenum alloyed cast iron with the DIN designation C-X 250 CrMg 153. With regard to this material it is stated in the journal "cav 8/86", pages 87 to 90 that as far as hardness and wear resistance are concerned it is only surpassed by pure metal oxides/carbides or also by hard surfacings, but that in comparison therewith it has the advantage of being machinable in the soft-annealed state. Apart from the customary casting materials GG-25, GGG-40.3, etc. which are also in current use, other corrosion-resistant special materials which are also suited for delivery media with a high content of solids are -according to this literature - an austenitic ferritic steel with the DIN designation C-X 3 CrNiMoCu 24 6 and a chill casting which is also chromium-molybdenum alloyed and has the DIN designation G-X 170 CrMo 25 2. Since all of these special materials are relatively expensive, their use has substantially been restricted to the impeller and apart from that - but only in practice - to component parts which, like the two rings of the split ring seal which is disposed on the front cover disk of the impeller in every centrifugal pump, turn out to be replacement parts which are especially susceptible to wear. Furthermore, the journal "Wekstofftechnik", Vol. 17, October 1986, pages 378 to 384, shows the results of tests in which a comparison is made between the aforementioned austenitic ferritic steel with the DIN designation G-X 3 CrNiMoCu 24 6 and a cast polyamide made by the activated anionic polymerization of monomeric laurinlactam and in which the destruction of the materials by cavitational loading is examined. In the preliminary period of the present invention this cast polyamide already proved to be a highly resistant alternative material to a high-quality nickel chromium steel when it was used for a wear disk in centrifugal pumps intended for delivering very abrasive media occurring in sugar industry.
This invention which is characterized by the patent claims solves the problem of providing a split ring seal of the stated type which is also very susceptible to wear in centrifugal pumps and in the case of which the knowledge gained in the construction of pumps is broadened with regard to a possible use of the aforementioned cast polyamide as an alternative material for other critical pump parts of this type.